Ceramic tiles are widely used to insulate underlying structures against the high temperature encountered in some applications. Examples of such applications include commercial furnace fiber linings, back decks on aircraft, and surface tiles for vehicles like the space shuttle. An aerodynamic vehicle such as the space shuttle or a supersonic aircraft is heated by friction to high temperature as it flies through the air, and especially as it re-enters the atmosphere from orbit. To insulate the airframe structure from the high temperature resulting from frictional heating, a portion of the outside of the skin of the aircraft may be covered by ceramic insulation that can withstand those high temperatures.
The ceramic insulation normally consists of ceramic tiles affixed to the exterior of the skin, which in turn is supported on the airframe. During flight, the outer surface of the ceramic insulation tile is heated to an extremely high temperature, but the interior surface of the tile remains much cooler because of the poor thermal conductivity of the ceramic material.
One known type of ceramic insulation is a porous ceramic made by pressing together fibers of one or more ceramic materials. The fibrous, porous ceramic is resistant to damage from thermal shock and thermal cycling, but it is relatively soft and can be damaged by external impact and wear forces. It is known to apply coatings to such surfaces to harden them and reduce the risk of damage from flying dust and debris.
The art has addressed these issues in a variety of ways. For example, a number of solutions have been offered that involve adding a surface layer to the ceramic body. In U.S. Pat. No. 4,093,771, a composition containing a solvent carrier, finely divided particles of a reactive glass frit, and an intermetallic compound such as boron silicides or silicon borides is sprayed onto the ceramic surface, dried, and sintered at 2225° F. The resulting surface layer, however, appears to be susceptible to chipping and peeling.
According to U.S. Pat. No. 5,079,082, an external coating of a glass or a glass and ceramic is applied to the surface of the ceramic insulation and fired. A diffuse, graded interface is formed, which partially overcomes the problems of spallation and surface separation associated with forming a separate distinct layer on the surface, but improvements are still necessary.
A recent advance has provided a surface protective treatment that results in a protected surface having no discernible separate coating layer. Rather, the protective agents are incorporated into the body of the ceramic. U.S. Pat. No. 5,702,761 thus provides for impregnating a slurry containing a ceramic powder and a binding agent into the surface of a tile, and thereafter drying the slurry and firing the tile to effect binding of the ceramic powder particles into the pores of the ceramic tile. A protected tile with increased hardness and impact resistance is produced thereby addressing problems of surface separation and spallation.
The protected tile of U.S. Pat. No. 5,702,761 has a perceived drawback in that at temperatures above about 2000° F., the silica binding agent in the tile devitrifies or crystallizes, leading to a diminution of desirable properties of the tile.
It would thus be desirable to provide a method for protecting ceramic bodies that would give the advantages of having no discernible surface layer, but that would be able to resist even higher in-use temperatures. Another object of the invention is to provide a protected ceramic body that can withstand temperatures above those at which the silica binding agents of the prior art vitrify, especially above about 2000° F.